The present invention generally relates to drive systems, such as drive systems involving a bit and a fastener, as well as a punch for forming a recess in the fastener.
Typical fastener drive system designs or geometries result in various surface contact patterns between the drive tool (i.e., bit) and fastener drive feature (i.e., recess). For example, some drive system geometries result in a “point” contact surface pattern, meaning that when the bit is rotated to initial contact with the recess (with near zero reaction torque), it contacts the recess at a point (or a plurality of points around the recess).
Other drive system geometries result in a “line” contact surface pattern, meaning that when the bit is rotated to initial contact, it contacts the recess at a plurality of lines. To place the bit inside of the recess in the fastener, there has to be some sort of gap between the bit and recess. As the bit is rotated, the gap between the bit and recess narrows until there is line contact with the sidewalls of the recess. Both point and line contact systems generate high stresses throughout the drive system and can also add to bit failure.
Still other drive system geometries result in an “area” contact surface pattern from the end of the bit to the top of the recess. Generally, an “area” contact surface pattern is more beneficial than a “line” contact surface pattern, and a “line” contact surface pattern is more beneficial than a “point” contact surface pattern.
However, even with regard to an “area” contact surface pattern, as bit-recess reaction torque (i.e., drive torque) increases, the drive bit geometry is elastically distorted (i.e., twisted and compressed), as well as the recess geometry (i.e., compressed), causing the bit-recess contact surface pattern to change and shift from the end of the bit toward the top of the recess. As the reaction torque increases, the surface contact pattern area tends to decrease, thus further increasing bit-recess contact stresses. The increased contact stresses at the top of the recess may damage the fastener finish (i.e., coating), and may lead to recess failure (ream-out). The increased contact stresses on the bit (and twisting) may cause premature wear, recess failure and fatigue failure.